1. Field of the Invention
The present invention relates to a vehicular constant speed cruising control device which is used in a vehicle that has an automatic transmission control device for automatically shifting the gears of the transmission.
2. Description of Related Art
For conventional vehicular constant speed cruising control devices, when constant speed cruising becomes difficult during overdrive cruising along upwardly sloping roads, speed is maintained by providing an overdrive cut signal to the automatic transmission controller to increase driving power. This prevents the occurrence of the commonly known "busy shift" in which the shift control of the vehicular transmission control device and the control of the constant speed cruising control affect each other.
Busy shift, which is a periodic fluctuation in the vehicular speed that occurs at fixed periods, occurs in the following way. Because vehicular speed declines when approaching an upwardly sloping road after overdrive constant speed cruising along a flat road, the constant speed cruising control device tries to maintain constant speed cruising by increasing actuator position and by adjusting the throttle valve to the open side. Because the throttle opening increases with the actuator position, in accordance with the transmission pattern, the automatic transmission control device down-shifts from overdrive. Accordingly, driving power increases, thus causing an increase in the vehicular speed. As a result, the constant speed cruising control device decreases actuator position and adjusts the throttle valve to the closed side to maintain constant speed cruising. The throttle opening decreases in connection with the decrease in the actuator position and thus, following the transmission pattern, the automatic transmission control device up-shifts to overdrive. Therefore, driving power goes down, causing vehicular speed to drop. In this way, vehicular speed is dropping again and hereinafter, the processes noted above are repeated again and again with the fluctuation in the vehicular speed occurring at fixed time intervals. This busy shift gives an unpleasant feeling to the passengers of the vehicle.
Japanese Patent Publication Hei-5-57933 discloses one method of preventing busy shifts in which the constant speed cruising control device prohibits up-shifts of the automatic transmission control device after the latter performs a down-shift from overdrive or the like. Also, the up-shift prohibition is released after a predetermined period of time lapses after the vehicular speed approaches the target cruising speed within a predetermined period of time.
However, for this method disclosed in Japanese Patent Publication Hei-5-57933, because up-shift is allowed after a predetermined period of time has lapsed after the vehicular speed reverts back to the target vehicular speed, then the following cycle occurs when up-shift is executed while the vehicle is still cruising along an upwardly sloping road: drop in the vehicular speed.fwdarw.increase in the actuator position.fwdarw.execution of down-shift.fwdarw.prohibition of up-shift.fwdarw.increase in the driving force.fwdarw.vehicular speed reverts back to the target level.fwdarw.release of up-shift prohibition after a predetermined period of time lapses.fwdarw.execution of up-shift .fwdarw.decline in the driving power.fwdarw.drop in the vehicular speed. Thus, a vehicular speed fluctuation cycle occurs at predetermined fixed periods and therefore, problems with vehicular passenger comfort still remain unsolved. In addition to this case, there were cases when the timing for releasing up-shift were inappropriate despite the fact that the driver manually prohibited overdrive.